Glazings having certain electrical, thermal or optical characteristics used as a heating glazing or an optical element, for example, are known. It is also known that glazings or optical elements may be provied these characteristics by coating the glazing with a metal oxide layer obtained by high temperature decomposition, followed by oxidation of a compound initially in the form of a powder distributed on the heated glazing or optical element. If the characteristics are to be uniform over the entire surface, it is necessary that any variation in thickness of the layer be as small as possible. In practice, the variation should not exceed 1% of the nominal thickness. Accordingly, as may be appreciated, the powder should be distributed with great precision.
A plate metering device providing an output in the form of a continuous, constant flow of powder in a disagglomerated and practically fluidized form is known and has been successfully used in such distribution. Such a metering device is described in French patent application No. 85 00052. As described, powder extracted at the output of the metering device is distributed on a substrate. The extraction and distribution is carried out in a manner to avoid, the extent possible, any compacting of powder during its transfer. If this precaution were not taken, irregularities in the thickness of the layer, reflected by anomalies in appearance, or in the optical, electrical and/or thermal properties, would be observed.
The extraction of the powder and its distribution on a substrate can be achieved by pneumatic ejectors. Several techniques and uses of the ejectors of the air jet type including a suction cone connected to its narrow end to the input opening of a tubular injector body are known. This form of ejector may include an injector body having an intake through which air for the entrainment of powder is injected. The intake may be located laterally of the body for communication into an annular chamber provided with a narrow annular gap. The annular gap may be located between an input opening and the end of a nozzle extending along the axis of the suction cone.
The injected air, at the gap output, comes out at the speed of sound creating a negative pressure at a nozzle input. Since the cone input is at atmospheric pressure, that is, it is not the situs of any negative pressure, a suction flow will be induced in both the cone and nozzle. The induced flow generally will be on the order of about 50% of the injected flow. With the high volumetric efficiency and almost a zero negative pressure at the input, it may be seen that the ejector will behave like a veritable amplifier with regard to disturbances that may be produced in the suctioned powder flow. The suctioned powder, itself, may act as an exciter. Thus, a disturbance at the input, for example, a variation in the concentration of powder in the suctioned mixture will be amplified and it will become more intense at the output with little opportunity for control. An ejector of this type, quite obviously, is unstable and not suitable for the fabrication of substrates coated with fine layers of material. Ths is particularly the case under circumstances that a desired precision of less than 1% is to be maintained.
Another form of ejector including an injection stage consisting of a venturi, and a suspension stage comprising an axial extension of the venturi is also known. This ejector functions by means of a suction of primary air and the mixture of air and powder within an input whose axis not only is perpendicular to that of the venturi but also comes out at the level of the nose of the venturi.
An ejector of this type permits a build up of great negative pressure at the input with only a slight flow. While the ejector is quite stable and, it would appear quite suitable for entraining the powder as a suspension in an air carrier, the range of stability of the ejector is very narrow. Further since the range of stability is imposed by the diameter of the venturi it cannot be modified for a given injector. Further still, the flow delivery is extremely low. Finally, the ejector runs the risk of clogging of powder and the powder layer that is formed on the venturi causes destabilization of the ejector when it becomes thick enough. This condition is thought to arise because of the location of the venturi nose in the path of the suctioned air-powder mixture.